Prior art inertia latches in passenger car seat back hinges employ an inertia element which blocks forward tilting of the seat back under emergency vehicle deceleration while accommodating forward tilting of the seat back for rear seat passenger ingress and egress without the necessity for actuating a manual release lever. The inertia element employed in such system is responsive to gravity as the seat back moves forward to move the latch to a release position and is responsive to inertia deceleration forces to retain and increase its latching attitude upon sudden deceleration. This is accomplished by locating the center of gravity of the inertia element with a horizontal displacement relative to a pivotal mounting on the seat back providing a gravity actuated bias in an unlatching direction, and with a vertical displacement providing a latching bias responsive to inertia incident to vehicle deceleration. With the seat back in normal user position such inertia element is cammed to a latching attitude overcoming gravity unlatching bias. Upon downward inclination of the vehicle when parked on a hill the inertia elements center of gravity shifts relative to its pivot in a direction decreasing the effective moment arm for gravity unlatching actuation. When parked on a hill with substantial downward inclination, e. g. over 13.degree., the effective moment arm for gravity release in such prior art constructions failed to dependably accommodate forward seat tilting without actuation of a manual release lever.
Specifications for inertia latches, under both commercial and governmental FMVS standards, include not only an acceptable range of deceleration values which will retain the latching attitude of the inertia element to block forward seat back tilting but also a requirement for dependable latch release under gravity actuation with the vehicle parked nose down at 15.degree. and the seat back adjusted to a standard position. Accordingly, various means were sought to extend the operative nose down parking range without impairing the operation of the inertia latch in effectively locking under such commercial and government specifications.